fix(spanner): allow toStruct to decode value into struct with unequal…

… number of fields compared to Spanner row fields

spanner/row_test.go

@@ -559,7 +559,7 @@ func TestToStructInvalidDst(t *testing.T) {
 			errDupGoField(&struct {
 				PK1 string `spanner:"STRING"`
 				PK2 string `spanner:"STRING"`
-			}{}),
+			}{}, "STRING"),
 		},
 		{
 			"Decode row as STRUCT to Go struct with no field for the PK column",

spanner/value.go

@@ -2872,8 +2872,8 @@ func errNilSpannerStructType() error {
 }
 
 // errDupGoField returns error for duplicated Go STRUCT field names
-func errDupGoField(s interface{}) error {
-	return spannerErrorf(codes.InvalidArgument, "Go struct %+v(type %T) has duplicate fields for GO STRUCT field", s, s)
+func errDupGoField(s interface{}, name string) error {
+	return spannerErrorf(codes.InvalidArgument, "Go struct %+v(type %T) has duplicate fields for GO STRUCT field %s", s, s, name)
 }
 
 // errUnnamedField returns error for decoding a Cloud Spanner STRUCT with
@@ -2930,7 +2930,7 @@ func decodeStruct(ty *sppb.StructType, pb *proto3.ListValue, ptr interface{}) er
 	for _, f := range fieldNames {
 		sf := fields.Match(f)
 		if sf == nil {
-			return errDupGoField(ptr)
+			return errDupGoField(ptr, f)
 		}
 	}
 	seen := map[string]bool{}
@@ -3009,10 +3009,9 @@ func decodeStructArray(ty *sppb.StructType, pb *proto3.ListValue, ptr interface{
 
 func getAllFieldNames(v reflect.Value) []string {
 	var names []string
-	s := v
-	typeOfT := s.Type()
-	for i := 0; i < s.NumField(); i++ {
-		f := s.Field(i)
+	typeOfT := v.Type()
+	for i := 0; i < v.NumField(); i++ {
+		f := v.Field(i)
 		fieldType := typeOfT.Field(i)
 		exported := (fieldType.PkgPath == "")
 		// If a named field is unexported, ignore it. An anonymous